1. Field of the Invention
The present invention concerns in general magnetic resonance tomography (MRT) as used in medicine for examination of patients. In particular, the invention concerns a method for the automatic segmentation of flow images acquired in magnetic resonance tomography for the depiction of, for example, arterial systems traversed (flowed through) with blood.
2. Description of the Prior Art
MRT is based upon the physical phenomenon of nuclear magnetic resonance and has been successfully used as an imaging modality in medicine and biophysics for over 15 years. The subject is exposed to a strong, constant magnetic field, cause the nuclear spins of the atoms in the subject, which were oriented randomly, to align. Radio-frequency energy can now excite these “ordered” nuclear spins to a specific oscillation. This oscillation generates the actual measurement signal in MRT, which is acquired by means of suitable reception coils. The measurement subject can be spatially coded in all three dimensions, generally designated as spatial coding, by the use of non-homogeneous magnetic fields generated by gradient coils.
The acquisition of data in MRT ensues in k-space (frequency domain). The MRT image in the image domain is linked with the MRT data in k-space by Fourier transformation. The spatial coding of the subject, which spans k-space, ensues in all three directions by magnetic gradients, including a slice selection gradient (establishes a slice selection of the subject, typically the z-axis), a frequency-coding gradient (establishes a direction in the slice, typically the x-axis) and a phase coding gradient (determines the second dimension within the slice, typically the y-axis). Moreover, the selected slice can be sub-divided into further slices along the z-axis by phase coding.
Thus a slice is selectively excited, for example, in the z-direction, and a phase coding in the z-direction is simultaneously executed. The coding of the spatial information in the slice ensues via a combined phase and frequency coding, by means of both of these aforementioned orthogonal gradient fields which, in the example of a slice excited in the z-direction, are generated in the x-direction and y-direction by receptive already-cited gradient coils.
A possible form of a sequence for data acquisition in an MRT scan is depicted in FIGS. 6A and 6B. The sequence used is a spin-echo sequence. In this, the magnetization of the spins In the x-y plane is achieved by a 90°-excitation pulse. In the course of time (½ Te; Te is the echo time), it leads to a dephasing of the magnetic components which together form the transverse magnetization in the x-y plane Mxy. After a certain time (for example, ½ Te), a 180° pulse is emitted into the x-y plane, so that the dephased magnetization components are mirrored without changing the precession direction and precession velocity of the individual magnetization portions. After a further duration ½ TE, the magnetization components again appear in the same direction, i.e. a regeneration of the transverse magnetization occurs, designated as a “rephasing. The complete regeneration of the transverse magnetization is designated as a spin-echo.
In order to acquire data for an entire slice of the subject to be examined, the imaging sequence is repeated N times with different values of the phase coding gradient, for instance Gy. The temporal separation of the respectively excited RF pulses is designated as the repetition time TR. The magnetic resonance signal (spin-echo signal) is likewise sampled, digitized, and stored N times in every sequence repetition by means of Δt-clocked ADC (analog-digital converter) in equidistant time steps in the presence of the read-out gradient Gx. In this manner (according to FIG. 6b) a numerical matrix is created row-by-row (matrix in k-space, or k-matrix) with N×N data points. An MR image of the slice inquisition can be directly reconstructed with a resolution of N×N pixels from this data set via a Fourier transformation (a symmetric matrix with N×N points is only one example, asymmetrical matrices can be generated as well).
For example, the curve of the average velocity of the flowing medium in a specified vessel during a movement cycle (breathing, heart movement) can be determined by velocity-resolved flow measurements in MRT, or the velocity distribution can be determined in a cross-section of the traversed vessel area of interest, or even further characteristic flow magnitudes at a specific point in time of the movement. For example, the velocity curve of the blood in the aorta during a heart cycle (from systole to systole) is of great interest.
For such measurements, two data sets are acquired virtually simultaneously during the movement, i.e. within a cycle to be measured: an anatomical image series as well as a velocity-coded image series. Typically the image acquisition rate in both series is approximately 20 images per cycle. The simultaneity of the image acquisition is realized by alternatingly acquiring an image of the one series followed by an image of the other series. During the acquisition of the velocity-coded series a constant gradient is applied in the flow direction that is adapted to the diverse sequence parameters (repetition time, flip angle, etc.) as well as to the flow velocity in the vessel of concern, in order to achieve an optimal velocity resolution. The acquisition slice of both series typically is oriented perpendicularly to the vessels to be depicted. The additional (phase coding) gradient in the flow direction is therefore necessary in order to be able to associate a defined velocity with every voxel of the flowing medium based upon the velocity-dependent dephasing, represented by the intensity of the magnetic resonance signal.
Conventionally the velocity-coded image series has been depicted in the form of a phase-coded flow image series and evaluated together with the anatomical image series with aid of post-processing software, generally after the completion of the examination on the patient. A quantified flow measurement is thereby possible by means of MRT, but as of yet there is no automatic evaluation and instantly surveyable depiction as is available, for example, in ultrasonic imaging. In the prior art, the user can separately view the flow image series (phase-coded image series) as well as the anatomical image series and optionally colorize the phase-coded image data set with non-standardized color palettes with the help of the post-processing software. The user can subsequently mark a flow region in both image series with a contour (border or outline), whereupon characteristic flow quantities of these marked areas are calculated and graphically presented.
The drawing of the contour can only ensue manually with a mouse. The evaluation of the marked areas must also be started manually. Moreover, a depiction of the velocity distribution in the form of a histogram—as is for instance standard with a Doppler ultrasonic examination—is presently not possible.